This invention relates generally to a coextrusion apparatus for producing a multilayer film or sheet of diverse thermoplastic materials. The apparatus includes a slit die having a slotted die opening through which the materials are extruded as a multilayer film or sheet. The die has a coathanger-type expansion chamber.
A feedblock assembly is incorporated into the die, the assembly including feed lines for conveying thermoplastic melt streams from the extruders to the feedblock inlet for forming a multilayer film or sheet as the layers combine upon exiting the feedblock, expand transversely in the chamber and extrude through the die opening.
More particularly, the invention relates to such coextrusion apparatus in which the die has a cavity for the reception of a feedblock formed as part of the feedblock assembly such that combining of the layers upon exiting the feedblock takes place within the die itself and is close as possible to the entrance to the expansion chamber. The feedblock comprises a plurality of slotted, layer distribution passages opening into the expansion chamber via a shallow entrance section, the passages comprising mutually spaced apart openings lying parallel to the slotted die opening. And, the feedblock is replaceable to accommodate selected polymer matchups, different numbers of multilayers, changes in layer thickness, layer geometry, etc.
Moreover, the invention provides for a highly effective end encapsulation of the multilayer film or sheet, and includes external adjustment to control the thickness, profile and distribution of the skin layers.
Coextrusion systems for forming multilayer film or sheets of thermoplastic materials are generally known, as shown for example in "Modern Plastics", August 1983 pages 22 to 26, and in "Plastic", August 1988, page A33. In both systems, between several extruders which each generates a polymer melt, and the slit die, a feedblock is located outside the die for combining the thermoplastic layers upstream of the die expansion chamber which is generally of the coathanger-type. Distributor pins, adjustable flow dividers, flaps, restrictor bars, or the like, are provided to control the thickness and distribution of the thermoplastic materials passing through metering channels and thus the thickness of each layer at the feedblock.
The feedblock, located upstream of the die, is generally accessible from all sides and is provided with means to control the different layer thicknesses of the polymer melts, but requires a specific feedblock for each desired layered structure from the standpoint of material and arrangement. The feedblock itself or significant elements thereof may therefore be replaced, and adjustments can be made for specific fluctuations and melt temperatures and/or viscosities of the individual layers which results in fluctuations in the layer structure of the final product.
Such adjustments to correct for fluctuations in temperature, viscosity, etc. will, of course, require significant experience on the part of the operator. Since the layer of thickness of each layer of polymer melt can be modified, the adjacent layers are always affected and consequently the entire layer structure is affected so that an unequivocal correction requires a very precise coordination of all existing possible corrections, a condition that poses continually higher demands on the experience of the operator as the number of layers increases. And, when extruding a multilayer sheet of a significant number of layers, the feedblock becomes continually more demanding and more costly.
Another drawback of the prior art coextrusion systems is that in the area of combining different polymers into the structure to be produced in the area of spreading the polymers into their final width and squeezing the layers to their final thickness in the coathanger die, the feedblock requires a channel of noticable length for reasons of stability. However, it has been demonstrated that not all layer structures of polymer melts necessarily flow strictly laminarly in such a channel but exhibit turbulence producing interfacial instability but cannot reform in the expansion chamber and therefore produce an undefined final product. Such a condition cannot be corrected by external adjustments made by the operator.
U.S. Pat. No. 4,619,802 discloses an extrusion apparatus which avoids some of the aforementioned drawbacks. The feed section to the feedblock is replaced by permanent channels in the die arranged in the flow direction of the polymer melt at the upstream side of the die. The feedblock, containing continuous flow channels, is removably inserted into a recess provided in the die, and the feedblock has an adjustable flow divider located between the flow channels to control the layer of thickness of the individual layers of polymer melt. Adjustment of the flow divider, however, affects both the adjoining layers of the entire layer structure.
In this prior art arrangement, the outlet of the removable feedblock is located as close as possible to the die manifold in which the layered melt stream expands in a direction parallel to the die slit while being simultaneously squeezed in a direction perpendicular thereto.
However, the drawback with this arrangement is that because of the adjustable vanes used to control the layer thickness the number of layers of the formed multilayer sheet is limited unless a multi-channel die is used. Moreover, such a coextrusion die requires a rather deep (measured in the flow direction) die in order to accommodate both a coathanger type expansion chamber, a removable feedblock and the permanent channels in the die leading from the extruders to the feed block through the rear section of the die. Moreover, the type of feedblock used is restricted in that the number and distribution of the polymer melts along the passage in the removable feedblock must be associated with the fixed feed channels at the rear section of the die.
The limitation of the number of polymer melts to be combined in the feedblock because of the means used to mechanically control the layer thicknesses, as in the prior art systems discussed above, can be avoided by the provision of a feedblock described in "Plastics Engineering", March 1974, pages 65 to 68, FIG. 3. With the feedblock method, it is possible to make a number of different layered structures by changing the feedports ahead of the single manifold die, provided that the individual extruders are sized for the desired product structure. The extruders are connected to feedblock manifolds ahead of the die. The individual manifolds introduce the various polymer melts into a feedport module which positions the polymer streams into the desired layer sequence. The combined substances are then channeled into the die.
However, combining does not take place within the die, and the feedblock has no means to mechanically control the layer distribution and thickness of the individual layers of polymer melt, such as the outer or skin layers especially when such layers are to be very thin. The outer layers are not only subjected to homogenization with the adjoining inner layers, but skin layer non-uniformity may develop due to the shear/stress exhibited between that layer and the wall of the passage through which it travels, or between the skin layer and the wall of the expansion chamber.
Another problem not addressed by the prior art is the automatic shrinkage at the sides of the laminate after extrusion through the die. Every multilayer film or sheet of polymer generated by a slit die exhibits an undesired edge deformation, which is usually eliminated by suitable trimming. In so doing, however, there is a significant accumulation of waste which should be able to be again fed to the production process of the multilayer polymer film or sheet. However, due to its composition of all layers of the product produced before, the waste can be suitably recycled only by using specific measures, as described for example in German published application 36 04 004.